mon/HealthMonitor: avoid MON_DOWN for freshly added Monitor

mon/HealthMonitor: avoid MON_DOWN for freshly added Monitor

In testing, we often have the scenario where cephadm has created a
cluster but doesn't add more monitors until well past
mon_down_mkfs_grace. This causes useless MON_DOWN warnings to be thrown
which fails QA jobs. Avoid this situation entirely by giving a
reasonable grace period for a monitor added to the MonMap to join
quorum.

Fixes: https://tracker.ceph.com/issues/73934
Signed-off-by: Patrick Donnelly <pdonnell@ibm.com>